A wavelength multiplexing technology (WDM: Wavelength Division Multiplex) has recently been exploited not only to a backbone network, but also to a local area network in order to satisfy a future information demand and a new service, with development of an information-communication society by the Internet. However, in connection with the local area network, it has been common to utilize, not a wavelength multiplexing system (DWDM (Dense WDM) system), which has been widely used in the backbone network, but a wavelength multiplexing system (CWDM (Coarse WDM) system) utilizing a wide range of wavelength, particularly in view of costs, as well as management and operation of the network.
In connection with a light source in the local area network, an existing light source has a technical limitation, and there has been made a study to make an attempt to increase further an amount of information, with development of an information-communication society by the Internet. Moreover, there is a need to reduce costs as low as possible, and consequently, it is necessary to make a study to avoid, as far as possible, use of an optical modulator, which is commonly used in a backbone network, and to apply a direct modulation of a semiconductor laser in a fundamental way, thus achieving a speed-up operation.
In response, there have been reported, as a configuration that utilizes a part of a cavity as a direct modulation region so as to permit increase in a relaxation frequency of oscillation, distributed reflector (DR: Distributed Reflector) type and distributed Bragg reflector (DBR: Distributed Bragg Reflector) type semiconductor lasers (see for example Non-Patent Document 1 and Non-Patent Document 2).
In addition, a conventional semiconductor laser is provided with a 1×3 multi-mode interference optical waveguide, a single optical waveguide that is provided at one end of the multi-mode interference optical waveguide to guide an optical wave, and three optical waveguides (a linear waveguide and a curved waveguide) that are provided at the other end of the multi-mode interference optical waveguide to guide a laser beam. The single linear waveguide and a part of two curved waveguides constitute an optical phase matching region to match a phase of a light on the respective front end surface, and a resonator is formed between the rear end surface and the front end surface (see for example Patent Document 1).
A conventional optical waveguide device comprises a first optical waveguide, a second optical waveguide that is formed of a different material of the first optical waveguide or has a different structure from the first optical waveguide, and is connected to the first optical waveguide, and a 1×1 multi-mode interference waveguide that is formed by increasing the width of the first optical waveguide and the second optical waveguide in a vicinity of a connection interface between the first optical waveguide and the second optical waveguide, in order to permit easy and reliable inhibition of influence of a reflected light in a connection interface (see for example Patent Document 2).